In vivo operational fascicle lengths of vastus lateralis during sub-maximal and maximal cycling.

Instantaneous contractile characteristics of skeletal muscle, during movement tasks, can be determined and related to steady state mechanical properties such as the force-length relationship with the use of ultrasound imaging. A previous investigation into the contractile characteristics of the vastus lateralis (VL) during cycling has shown that fascicles operate on the "weak" descending limb of the force-length relationship, thus not taking advantage of the "strong" plateau region. The purpose of this study was to investigate if VL fascicle lengths change from sub-maximal to maximal cycling conditions, and if maximal cycling results in VL fascicle lengths which operate across the plateau of the force-length relationship. Fifteen healthy male subjects (age 20.9+/-1.8yr, wt. 67.0+/-6.3kg, ht. 176.7+/-7.2cm) were tested to establish the maximal force-length relationship for the VL through ten maximal isometric contractions at various knee angles. Subjects then cycled on an SRM cycle ergometer at cadences of 50 and 80 revolutions per minute at 100W, 250W, and maximal effort. Fascicle lengths were determined at crank angles of 0, 90, and 180 degrees . Fascicles operated at or near the plateau of the maximal force-length relationship for maximal cycling, while operating on the descending limb during sub-maximal conditions for both cadences. However, when comparing the fascicle operating range for the sub-maximal cycling conditions to the corresponding sub-maximal force-length relationships, the VL now also operated across the plateau region. We concluded from these results that regardless of cycling effort, the VL operated through the ideal plateau region of the corresponding force-length relationship, hence always working optimally. We hypothesize that this phenomenon is due to the coupling of series elastic compliance and length dependent calcium sensitivity in the VL.

Microstructural damage in arterial tissue exposed to repeated tensile strains.

OBJECTIVES: Vertebral artery (VA) damage has been anecdotally linked to high-speed, low-amplitude spinal manipulative treatments (SMTs) of the neck. Apart from a single study quantifying the maximum stresses and strains imposed on the VA during cervical SMT, there are no data on the mechanics of the VA for this treatment modality, and there is no information on the possible long-term effects of repeat exposure to cervical SMT. The purpose of this study was to quantify microstructural damage in arterial tissue exposed to repeat strain loading of a magnitude similar to the maximum strains measured in the VA during high-speed, low-amplitude cervical SMT. METHODS: Twenty-four test specimens from cadaveric rabbit ascending aorta were divided into 2 control groups (n = 12) and 2 experimental groups (n = 6 each). Specimens were exposed to 1000 strain cycles of 0.06 and 0.30 of their in situ length. A pathologist, blinded to the experimental groups, assessed microstructural changes in the arteries using quantitative histology. Pearson chi(2) analysis (alpha = .05) was used to assess differences in tissue microstructure between groups. RESULTS: Control and 0.06 strain tissues were statistically the same (P = .406), whereas the 0.30 strain group showed microstructural damage beyond that seen in the control group (P = .024). CONCLUSIONS: We conclude that cadaveric rabbit arterial tissue similar in size and mechanical properties of that of the human VA can withstand repeat strains of magnitudes and rates similar to those measured in the cadaveric VA during cervical SMT without incurring microstructural damage beyond control levels.